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Tesi Leonardo Ravizza Dipartimento di Scienze della Terra e dell’Ambiente Direttore: Prof. A. Di Giulio Corso di Laurea in Scienze Geologiche GEOTHERMOMETRIC APPLICATION ON THE GRAPHITE OF YAMATO 74123 UREILITE Relatore: Prof. Maria Chiara Domeneghetti Correlatrice: Dr.ssa Anna Barbaro Tesi di Laurea in Scienze Geologiche di LEONARDO RAVIZZA matr. 454180 Anno Accademico 2019/2020 Dichiaro che la presente tesi sperimentale di Laurea Triennale in Scienze Geologiche è stata svolta sotto la mia guida e su tema da me assegnato, presso il Dipartimento di Scienze della Terra e dell’Ambiente dell’Università degli Studi di Pavia. Prof. Maria Chiara Domeneghetti 2 TABLE OF CONTENTS ABSTRACT 4 1 INTRODUCTION 5 1.1 Ureilites 5 1.2 Yamato meteorites 8 2 SAMPLE 10 2.1 Sample description by optical microscopy and scanning electron microscopy (SEM) 10 3 EXPERIMENTAL METHODS 15 3.1 Micro-Raman Spectroscopy (MRS) analyses 15 4 DISCUSSION 18 5 CONCLUSIONS 19 6 REFERENCES ACKNOWLEDGEMENTS 24 3 ABSTRACT The aim of this bachelor thesis is the study of graphite in Yamato 74123 sample. I obtained geothermometric data by micro-Raman spectroscopy of non-polished graphite in Y- 74123 ureilite from the collection of the Natural History Museum Vienna. In this sample, graphite shows a G-band peak center between 1579 and 1582 cm-1 and a full width at half maximum (FWHM) between 11 and 16 cm-1 corresponding to a crystallization temperature of graphite with values ranging between 1265 and 1334 (± 120) °C. This temperature range is slightly higher than the previously estimated crystallization temperatures reported for graphite in some Almahata Sitta (AhS) ureilites. Recent works by X-ray diffraction on Y-74123 and on NWA 7983 ureilites showed that the graphite is associated with abundant nano and micro diamond and argued that this assemblage formed in an impact event. I investigated by micro-Raman Spectroscopy the same Y-74123 sample and propose that the temperature obtained on the graphite on this sample [1100-1300 °C (±120 °C)] also records an impact event. Although this temperature is very close to those obtained on ureilites using geothermometry applied to silicates, and interpreted to record igneous processing, the observation that the graphite in this sample is completely recrystallized to nanometric grain size, indicates that it records a shock event. 4 1 INTRODUCTION The main aim of this bachelor thesis is to investigate the graphite from the ureilite sample Yamato 74123 by micro-Raman Spectroscopy to obtain the temperature recorded by this carbon phase in order to constrain its thermal history. Scanning Electron Microscopy and micro-Raman Spectroscopy were carried out at CEASC (Centro di Analisi per la Certificazione) of University of Padua and at the Earth and Environmental Science Department of University of Pavia respectively. The project was performed in collaboration with the Natural History Museum Vienna and with Cyrena A. Goodrich of Lunar and Planetary Institute in Houston. 1.1 Ureilites Ureilites represent the second largest group of achondrites and their evolutionary history in still under discussion among the scientific community. In particular, in the last years there is a huge debate about the origin of carbon phases contained in these meteorites. As reported by Goodrich et al. 1992, ureilites appear to be highly fractionated igneous rocks either magmatic cumulates (Berkley et al., 1976; 1980; Goodrich et al., 1987a) or partial melt residues (Boynton et al., 1976; Scott et al., I992) and thus the products of planetary differentiation processes. These assumptions were made considering the mineralogy, the textures and fabrics, the lithophile element chemistry, and some aspects of Sm-Nd isotopic systematics (Berkley et al. 1976) observed in these meteorites (Goodrich et al. 1992). Ureilites mainly differ from the other stony meteorites and in comparison with chondrites they are enriched in Mg but depleted in metal, troilite, alkaline elements. They contain large olivine grains and few smaller low Ca clinopyroxene (pigeonite) aggregates. Minor phases are kamacite (Ni content being 1.5-4 %), troilite FeS (1-2%), chromite (1-2%) 5 and carbon material (8.5%). Carbon material is represented by diamond (usually with stacking disorder), graphite and organic material. Ureilites present different shock levels, which are very important for constraining the evolutionary history experienced by these meteorites. Shock level determination in meteorites was proposed by Stöffler et al. (1991, 2018). These authors proposed six stages of shock, from S1, low shock degree to S6, high shock degree, based on shock effects in the texture of olivine and plagioclase (e.g. extinction, fractures, presence of glass or high pressure silicate phases) observed on thin sections. The shock effects in olivine have been correlated with level of shock in diamond and carbon phases in ureilites by Nakamuta et al. (2016). A typical feature of shock degree from S3 to S6 is the presence of melt veins (Rubin et al. 1997; Stöffler et al. 1991, 2018). The process through which carbon phases formed has been debated and is still controversial among the scientific community. One of the main questions about ureilites is why they contain large amount of noble gases concentrated in carbon veins: diamond is shown to be one of the noble gas carriers, while graphite is usually gas-free. The first hypothesis about the formation of diamonds offers an answer to this question: the formation of diamonds by chemical vapour deposition (CVD). In typical chemical vapour deposition, the substrate is exposed to one or more volatility chemistry precursors which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by- products are also produced. Fukunaga et al. (1987) synthesized using an experimental apparatus diamonds by CVD from gaseous mixture of H2 and CH4 including argon (Ar). Argon entrapped in diamond was examined by mass spectrometry to compare the results with data obtained from ureilites samples. The comparison showed many differences between Ar content in diamond and in graphite. This was consistent with the observations by Göbel et al. 6 (1978) who excluded graphite as a carrier of noble gases in ureilites. Few years later, a further hypothesis was proposed about the CVD formation of diamonds in ureilites stating that diamond with noble gas content was probably formed from the solar nebula (Fukunaga et al., 1987). Another interpretation about the presence of noble gases content in diamonds was proposed by Lewis et al. (1987), suggesting the primary formation of diamond by stellar condensation, based on the study of many ureilitic samples of Allende, Murchison and Indarch meteorites. The second hypothesis proposed that diamond formation in ureilites took place under static high-pressure conditions in the deep interior of a ureilite parent body (UPB) similar to the Earth’s mantle diamonds. This hypothesis was proposed for the first time by Urey et al. (1956), investigating diamonds samples collected in the Canyon Diablo meteorite, which are collected in Arizona in the crater with the same name. These authors claimed that diamonds cannot be produced except under fairly high temperature and high pressure. Using thermodynamic data and the density of graphite (2.26 gcm-3) and diamond (3.51 gcm-3), they understood that the required minimum pressure at 1000 K and 1200 K are at 31,000 and 37,000 bars, respectively. This huge pressure can explain the formation of diamond in the deep interior of a big parent body, with dimension similar approximately to Lunar mass or greater (Urey, 1956). This hypothesis was assumed for Almahata Sitta ureilitic samples in two recent works by Miyahara et al. (2015) and Nabiei et al. (2018). Another investigation about diamonds in ureilites of Canyon Diablo showed a pronounced similar crystallographic orientation among diamond fragments (Lipschutz, 1964). These authors proposed the formation of diamond in ureilites as product of transformation of graphite after a shock event. Some experiments on meteoritic fragments of ureilites and on diamonds were carried out using X-ray diffraction and Raman spectroscopy (Nakamuta et al., 2016). In this work we performed Scanning Electron Microscopy (SEM) and micro-Raman Spectroscopy on the fragment Yamato 74123 and the petrographic characterization on a thin 7 section obtained from the same fragment (NHMV-L9822), belonging to the collection of the Natural History Science Museum of Vienna. 1.2 Yamato meteorites Yamato 74123 is an Antarctic meteorite that was found on 1974 by the Japanese expedition on the Yamato mountains. In Yoshida et al. (1971) the first discovery of Yamato Meteorites by an inland survey team of the Japanese Antarctic Research Expedition (JARE) in 1969 is reported. The members of the team collected three stones on the surface of the ice sheet in the south-eastern marginal area of the Yamato Mountains. They realized that these stones could have been meteorites and started to collect other possible meteorites. Back to Japan, the nine rocks collected in Antarctica were all identified as meteorites. In addition, Yoshida et al. (1971) proposed a mechanism by which meteorites concentrated in the area, involving the flow, structure, and ablation of the ice sheet (Fig.1). Fig.1 In the picture ice movement and the consequently meteorite concentration is shown. 8 This hypothesis was developed in the field in 1969 during the collection program (Yoshida et al. 2009). After 1969 further collections of meteorites in the Yamato Mountains were conducted on 1973 and 1974, when Y-74123 ureilitic fragment was collected, and on 1975 seasons. A project involving the collection of meteorites was formally incorporated as an important component of the work undertaken by the geology group within JARE from the 1975 and 1976 seasons onwards. 9 2 SAMPLE 2.1 Sample description by optical microscopy and scanning electron microscopy (SEM) The fragment of Yamato, Y-74123, and its thin section, NHMV-L9822, belong to the collection of the Natural History Science Museum of Vienna.
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